The present invention relates generally to systems and methods for controlling the cleaning of industrial filter systems, such as textile barrier filters of the bag filter types, including a plurality of filter houses, such as a plurality of industrial baghouses.
Baghouses are employed, for example, for air pollution control purposes to separate undesirable particulate matter from a gas stream, such as a boiler flue gas stream, by fabric filtration. Fabric filtration is however not limited to air pollution control, but also is employed in resource recovery applications where an object is to recover the particulate matter.
Such filtration is carried out in filter houses, known in the trade as baghouses, which include a plurality of fabric bag filters suspended, generally open-end down, within the baghouse. Particulate-laden gas is directed upwardly into each bag such that particulate matter collects inside the bags as a filter cake. Gas is forced to flow through the baghouse by either a blower fan or a suction fan, and accordingly there is a pressure drop across the filters depending upon their resistance to gas flow. As a filter cake accumulates on the bag surfaces, gas flow resistance increases, decreasing gas flow and increasing pressure drop, which must then be overcome by the fan. Accordingly, the bag filters are periodically cleaned to remove the accumulated filter cake.
At the outset, it should be noted that terminology in this particular art is not completely standardized, and what are in fact different elements are sometimes referred to by the same name. For convenience and clarity, the terminology employed herein will now be defined in the context of a description of the overall organization of a multiple baghouse fabric filter system.
A singe "baghouse", which is also referred to more generically herein as a "filter house", comprises one or more "compartments". Each "compartment" is an independent structure, and is normally separated from other compartments within the same baghouse by walls, and is controllably separated from common inlet and outlet gas ducts by dampers. Each compartment in turn includes multiple bags, typically arranged in rows and columns. One or more baghouses (or, more generically, filter houses) together comprises a "fabric filter system". When more than one baghouse is employed in a fabric filter system, the individual baghouses are connected in parallel in the gas stream. Various prior art descriptions of baghouses apply such terms as fabric filter, bag filter, or collector, which terms are not employed herein. Similarly, various prior art descriptions of what are herein referred to as "compartments" apply such terms as collectors, baghouses, modules, or units, which terms are also not employed herein.
By way of example, the following U.S. patents are identified for their disclosures of various forms of baghouses of the type here concerned: Lincoln U.S. Pat. No. 3,097,936; Garrett U.S. Pat. No. 3,411,929; Adair et al U.S. Pat. No. 3,630,004; Laliwall U.S. Pat. No. 3,735,566; Slakey Pat. No. 3,898,062; Bundy U.S. Pat. No. 4,113,449; and Apelgren U.S. Pat. No. 4,277,255. A related system, which employs a combination settling chamber and wire screen, rather than a bag filter, is disclosed in Dorfan U.S. Pat. No. 1,907,197.
As noted above, as a bag filter removes particulate, a dust cake develops on the surface of the filter fabric. Hence, as filtering proceeds, the filter resistance and thus pressure drop increases. Periodic cleaning is therefore necessary.
In order to maintain a baghouse in operation even while cleaning is taking place, it is conventional practice to provide a cleaning cycle whereby individual compartments within a baghouse are cleaned one at a time, while the remaining compartments of the baghouse remain on-line to continue filtering operation. The compartment being cleaned is taken off-line by the closing of appropriate dampers connecting the compartment being cleaned to the common inlet duct, the common outlet duct, or both. After each compartment is cleaned, it is then returned on-line, and the next compartment in sequence is cleaned, and so on, until all compartments of the bag house have undergone a cleaning cycle.
As will hereafter become apparent, the present invention is applicable to various types of cleaning methods, but the cleaning method with which the invention is most directly concerned is known as reverse air or reverse gas flow cleaning. Reverse air cleaning uses a sustained period of low velocity, high volume gas flow within the compartment being cleaned, which flow is opposite to the normal flow of particulate-laden gas. All the bags in an entire compartment are cleaned simultaneously by this method. Of the various prior art patents identified above, reverse air cleaning is disclosed in the patents to Lincoln No. 3,067,936, Garrett No. 3,411,929, Adair et al No. 3,630,004, nd Slakey No. 3,898,062.
While the method of reverse gas flow cleaning is described hereinafter in greater detail, at this point it may be noted that forced reverse gas flow during cleaning opposes the main gas flow through the system, and thus adds, during a portion of the cleaning cycle, even more pressure drop or resistance than is caused by filter cake accumulation alone. Thus, as a normal and essential part of a reverse gas flow cleaning cycle, system pressure drop goes through sequential peaks and minimums. Not only must the system blower be sufficiently sized to handle the peak pressure drop, but care must be taken to ensure that proper fan operation is not disrupted by the magnitude or rapidity of pressure drop changes.
Briefly, two other known general cleaning methods are "pulse jet cleaning" and "Shaker cleaning". "Pulse jet cleaning" employs a high-pressure burst of a compressed gas to literally blow the dust off the bags. Typically, one row of bags is cleaned at a time, and the cleaning of the various compartments is usually done independently. By way of example, of the various prior art patents listed above, forms of pulse jet cleaning are disclosed in the U.S. patents to Laliwala No. 3,735,566, Bundy No. 4,113,449, and Apelgren No. 4,277,255.
"Shaker cleaning" employs mechanical agitation of the bags. Usually, two or more rows of bags are cleaned simultaneously, and compartments are usually cleaned independently. By way of example, the system of the Garrett Pat. No. 3,411,929, identified above, employs mechanical shaking, in addition to reverse air flow, to remove accumulated particulate.
The present invention may also be employed with advantage in combination with either "pulse jet cleaning" or "shaker cleaning". In cases where the inlet dampers, the outlet dampers, or both, for a particular compartment being cleaned are closed, the resistance characteristics of a baghouse using these two cleaning methods are similar to those of a baghouse employing reverse air cleaning. The present invention can be employed with similar results.
In the context of the present invention, the definition of a "baghouse" or, more generically, a "filter house" as defined at the outset above may more specifically be defined, in the context of function, as a set of compartments served by a single cleaning system, e.g., of the reverse air flow type, and which typically is cleaned compartment-by-compartment in a predetermined sequence until all compartments of the baghouse have been cleaned. Thus, there are baghouses which include several physically-distinguishable units which might be termed individual baghouses, but yet are operated as one in that the compartments are all served by the same cleaning system and are cleaned individually in a compartment-by-compartment sequence until all are cleaned. Such an arrangement, for present purposes, is nevertheless considered to be a single baghouse. This definition of a baghouse in the context of cleaning function is important for best appreciation and understanding of the present invention.
Various methods have previously been employed for initiating a baghouse cleaning cycle to clean the individual compartments compartment-by-compartment. In cases wherein the rate of particulate accumulation is sufficiently predictable, simpler timer-based systems have been employed.
More typically, pressure drop across the system is monitored, either automatically or by an operator, and each baghouse cleaning cycle is triggered when the pressure drop reaches or exceeds a threshold. In a multiple baghouse system, all baghouses are parallel to each other. Thus, although pressure sensors may be located at individual baghouses, the pressure sensors necessarily respond to total system pressure drop, and the baghouse cleaning cycles are all triggered at approximately the same time as the triggering threshold is reached. When the first baghouse to actually reach the pressure drop threshold begins its cleaning cycle, the first of its compartments going off-line immediately causes the pressure drop across the other bag houses to increase, immediately causing their triggering thresholds to be exceeded.
In other systems, the baghouses have been operated entirely independently, with no attempt to coordinate their cleaning cycles as in an oveall fabric filter system.